Conventionally, in an electrical power generating alternator in an automotive vehicle, diodes are used as rectifiers which convert a generated alternating current to a direct current to charge a battery.
As disclosed in Patent Literature 1, a rectifier using a diode has: a lead electrode connected to a terminal on an upper surface of the diode; and a base electrode connected to a terminal on a lower surface of the diode. The package of the rectifier is composed of the base electrode and is fixed to an electrode plate of the alternator through solder or by press-in. For an alternator, as many as six or twelve rectifiers need to be fixed on the electrode plate of the alternator. Therefore it is important that the rectifiers be readily fixed to the alternator in assembly steps of the alternator.
A rectifier using a diode causes a large loss because a diode has a built-in potential as shown in FIG. 24.
In contrast, a rectifier using synchronous rectification by a metal oxide semiconductor field effect transistor (MOSFET) has a small loss because a MOSFET does not have a built-in potential so that a forward current rises from a voltage of 0 V. That is, since the same amount of current flows through a MOSFET by applying a lower voltage compared to a diode, the loss caused by a MOSFET is smaller than that of a diode. FIG. 24 shows forward current-voltage characteristics of a rectifier using MOSFET according to the present invention and forward current-voltage characteristics of a conventional diode rectifier using diode.
Patent literature 2 and patent literature 3 disclose MOSFET rectifiers for use in an alternator. In those MOSFET rectifiers, a MOSFET chip is mounted on a square package (see FIG. 3 of patent literature 2 and FIG. 7 of patent literature 3). A MOSFET has a pair of main terminals (source and drain terminals) and a gate terminal. In a wiring step of package assembly it is necessary to recognize the position of the gate terminal to which a wire is to be connected. If a MOSFET chip is mounted on a package having a square shape, it is easy to position the MOSFET chip in a rotational direction around a rotation axis of the package, thus steps for wiring are easy.